Hypothyroidism in the newborn may result from absence or abnormal development of the thyroid gland, destruction of the thyroid gland, failure of stimulation of the thyroid by the pituitary gland, or defective or abnormal formation of thyroid hormones. Incomplete development of the thyroid is the most common defect and occurs at a rate of 1 out of every 3,000 births. Girls are affected twice as often than boys. Most affected infants have few or no symptoms, because they only have a mild deficiency of thyroid hormone. However, infants that are profoundly hypothyroid often have a distinctive appearance. The affected infant may have puffy-appearing face, a dull look, and a large tongue that protrudes from the mouth, which is usually kept open. This appearance usually develops as the disease progresses. The child may have dry, brittle hair, low hairline, and jaundice (yellow eyes and skin). Feeding is poor and the infant may choke frequently. Constipation is common. Affected children do not cry much, sleep a lot, are sluggish and inactive. Muscle tone is usually decreased. If untreated, even mild hypothyroidism can lead to severe mental retardation and growth retardation. Development is delayed early on, often indicated by failure to meet normal milestones. The appearance of teeth may be delayed. These problems become more severe as the child ages. Growth failure in terms of body length is noted very early.
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